Ordinary matter consists of atoms, and atoms are made of nuclei and electrons. But in a neutron star, gravity compresses matter so strongly that electrons are squeezed into protons, and almost everything turns into a dense packing of neutrons. It's like one giant atomic nucleus the size of a city. What if you squeeze even harder? Then neutrons start to break apart into their constituent parts — quarks. You get something like a hot 'soup' of free quarks that exists only in the deepest interiors.
The authors of the study tried to model this transition from neutron 'porridge' to quark soup. They based their approach on a theory similar to the description of superconductivity: quarks can pair up into diquarks. But pairs alone are not enough. To keep the star from collapsing under its own weight, additional stiffness is needed. This comes from the particles that carry the interaction — vector mesons. It's like adding not only flour and water to the dough, but also hard nuts that prevent it from spreading. As a result, the equation of state, the law linking pressure and density, becomes steep enough to support a mass of more than two Suns.
The calculations showed that if a neutron star is heavier than about two Suns, its central density exceeds four nuclear densities. That's like each grain-of-sand-sized cube weighing a billion tons. The radius of such a star is about twelve kilometers, and inside hides a quark core. These numbers agree well with actual observations: the most massive known neutron stars fit this description perfectly.
as written by the authors arXiv:2605.05816
We discuss the properties of the extended three-flavor quark-meson diquark (EQMD) model as a renormalizable low-energy effective model for QCD. The effective degrees of freedom are quarks, scalar- and pseudoscalar mesons, diquarks, vector- and axial-vector mesons. We calculate the equation of state (EoS) in the mean-field approximation at $$T=0$$ imposing charge neutrality for electric and color charges. We match the EoS with a low-density nuclear equation of state. We discuss how the choice of parameters in the model affects the EoS and thereby the mass-radius for hybrid stars. We show that it is possible to construct hybrid stars whose masses and radii are in agreement with recent astrophysical observations and perturbative QCD (pQCD). The addition of vector and axial vector mesons to the quark-meson diquark is essential, since it makes the EoS sufficiently stiff for intermediate densities. Our results suggest that stars with a mass larger than $$M\sim2M_{\odot}$$ have a quark core with a central density $$n_B\geq 3.9n_{\rm sat}$$, where $$n_{\rm sat}\approx0.165$$fm$$^{-3}$$ is the saturation density. The speed of sound has a double-peak structure and relaxes to the conformal limit from above for large baryon chemical potentials $$\mu_B$$. This structure is caused by the decrease in the mass of the $$s$$ quark as $$\mu_B$$ increases.
The density at the center of such a star is so high that a teaspoon of its matter would weigh more than all the mountains on Earth combined.
In Robert Forward's novel 'Dragon's Egg', life on a neutron star is described, where creatures made of nuclear matter live a million times faster than humans.